The field of the invention is systems and methods for microelectronic packaging. More particularly, the invention relates to the study of electromigration in micro-scale solder volumes, and the characterization of three-dimensional solder microstructures using micro x-ray computed tomography. The systems and methods utilize the observation of the evolution of three-dimensional microstructures in situ under a wide range of characterization modes.
Observing the microevolution of a sample is useful in many applications, for example microelectronic packaging. In microelectronic packages, an array of interconnects which are both electrical and mechanical have been used between the die and the package substrate as well as between the package substrate and a printed circuit board (PCB) to which the package is mounted. In flip-chip architectures, solder volumes with individual diameters of approximately 50 μm have been used to mount a die to a substrate. In mounting a package to a PCB, a ball-grid-array (BGA) of solder volumes, with individual diameters of approximately 500 μm, has been used. One significant long-term reliability concern with micro-scale solder volumes involves electromigration induced damage of the solder interconnects, established, in part, by the small conductor cross section within these architectures. Electromigration is the phenomenon of material migration under an applied electric field. The dominant electrical carriers are electrons, which move from the cathode toward the anode and cause a momentum exchange with thermally activated metal ions toward the anode yielding an “electron wind” force. It has been observed that at high current densities the electron wind force is substantially greater than, and opposite to, the electrostatic force, and thus is the dominating force on metal ions in the conductor.
A relatively small cross section implies high current density for a particular applied current magnitude. This effect may be enhanced by current crowding, thermomigration, the high concentration of vacancies in the low-melting temperature solder systems leading to self diffusion induced voiding, the rapid diffusion of, for example, copper (a common substrate metal) through the solder by interstitial mechanisms even at low temperature, and current induced Joule heating. Joule heating may cause aging effects on the solder grain structure and the formation of substrate-solder intermetallic compounds both at the interface and in the solder interior. Due to the highly anisotropic nature of tin, the crystallographic influence at small volumes is enhanced as the number of grains is reduced. It has been observed that solder interconnects with the c-axis parallel to the electron flow experience a much higher consumption rate of the under-bump-metallization (UBM) and reduced life.
With the above effects in mind, failure at the solder level within real packages is extremely complex. The microstructural evolution in an individual solder volume operating in an industrial package will conceivably depend on the electrical interconnectivity of the solder interconnects, the temperature and thermal gradient in the package, evolution in neighboring solders, the solder size and composition used, the substrate chemistry, and the thermal processing and heat treatment of the package and/or solder. Although previous research on electromigration induced material evolution in solders has been performed on actual packages or package-like testing structures, tests may also be conducted using individual solder volumes. Testing of individual solder volumes has the advantage of effectively reducing peripheral influences in the test and isolating material and environmental effects. However, the frailness of the single micro-joint in a free-standing butt-joint makes it extremely challenging to perform characterization, imaging, and testing.